By Ben Ellison
Where’s Rescue 21?
Well, you’ve got me looking at charts more carefully now; so what the heck does Mean Lower Low Water mean? J.H., via e-mail
Most of the United States has semidiurnal (or twice daily) tides, and one drains lower than the other, a phenomenon splendidly labeled Diurnal Inequality. At some point NOAA decided to be more conservative and chucked each day’s higher low (watch the twist) out of their datum calculations, leaving Mean Lower Low Water. The greenish intertidal area on every U.S. chart got a little larger! Still—and here’s the bottom line, so to speak—you can be in a place one day where the chart says six feet deep and it really is six feet at low tide. But a few days later, probably near a full or new moon, it might be three feet deep. A tide table, also based on MLLW, should show a minus-three-feet low that day.
Think about datums around bridges, too. NOAA uses Mean High Water as its Height Datum, but, whereas half the high tides exceed the mean, there are still many times when there’s less height than charted. Thankfully most bridges have physical tide/clearance gauges to help you deal with this twist. Note that if you voyage outside the United States you’ll find a variety of datums, almost all more conservative than ours. Canada, for instance, bases soundings on Low Astronomical Tide, the absolute Epoch minimum, which is why you’ll never see a minus sign on its tide tables. There are also Horizontal Datum issues, which can twist you up if you’re plotting GPS coordinates on certain foreign charts (discussed in last month’s column). Finally, while much of this detail is noted on paper charts but not on electronic ones, a tool like graphic tide software can more than make up for the deficiency, especially if you’re aware of the twists.
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This article originally appeared in the April 2005 issue of Power & Motoryacht magazine.